The present invention relates to circuits for controlling the operation of AC signal processing circuits used in television receivers and, more particularly, to circuits for defeating the operation of such processing circuits while simultaneously controlling the nature of the signal developed at the output thereof.
Television receivers frequently include circuits designed for processing one or more input AC signals to develop a desired output AC signal. For example, in copending application Ser. No. 179,142, filed Aug. 18, 1980, now U.S. Pat. No. 4,296,435, and assigned to the assignee of the present invention, there is disclosed an AC limiter circuit responsive to an input rectified first derivative signal for developing a corresponding amplitude limited output gate signal, the output gate signal being used to operate a noise processing circuit. At certain times, it is desirable to defeat the operation of the noise processing circuit which may be accomplished by coupling a DC voltage having a particular level to the noise processor circuit in lieu of the output gate signal from the AC limiter. Therefore, in order to defeat the operation of the noise processing circuit, it is necessary to inhibit the development of the gate signal at the output of the AC limiter while simultaneously coupling a DC voltage having a controlled level to the noise processing circuit.
In considering the design of a control circuit capable of performing the above functions, a manually operable switch is preferably provided to allow the viewer to selectively enable or defeat the processed AC output signal, such as the previously referred to gate signal. Moreover, since the electronic circuitry used in modern television receivers is largely fabricated in the form of integrated circuit chips, it is necessary to interconnect the manually operable function selector switch with the pins of the integrated circuit chip embodying the AC signal processing circuit. This interconnection is preferably effected using a minimum number of integrated circuit chip pins since the number of input/output pins is a major factor in determining the cost of a chip. It is accordingly a primary object of the present invention to provide a novel control circuit responsive to a manually operable selector switch for defeating the operation of an AC signal processing circuit embodied on an integrated circuit chip while simultaneously controlling the nature of the signal developed at the output thereof. It is a further object of the invention to provide a control circuit of the foregoing type which is embodied on the integrated circuit chip together with the AC signal processing circuit and which interfaces with the manually operable selector switch through a single integrated circuit chip pin.